Method and apparatus for applying a liquid coating with an improved spray nozzle

ABSTRACT

Apparatus and methods for spray coating selected areas of a circuit board with a liquid coating material without coating regions of the circuit board to be left uncoated. In one mode of operation, a bead of liquid coating material dispensed from an elongated nozzle tip is atomized by directing jets of air pressurized against the bead to form a conical, swirling spray pattern which is sprayed on selected areas of a circuit board. In a second mode of operation, the non-atomized bead of liquid coating material dispensed through the elongated nozzle tip is applied on selected areas of the circuit board. In a third mode of operation, jets of pressurized air are directed against the bead of liquid coating material to create a soft swirl pattern, without atomizing the bead of coating materia. In a second embodiment of the invention, the bead of liquid coating material is dispensed from a nozzle without the elongated nozzle tip.  
     In another embodiment of the invention, the nozzle is located closely adjacent a valve seat at the end of a liquid delivery passageway to minimize a residual volume of coating material in a volume between the valve seat and the nozzle so as to minimize drip and dribble upon startup. In another embodiment of the invention, pressurized air is diverted to an exhaust port when operating in a non-pressure-assisted mode so as to minimize pressure spikes and consequent splatter when initiating a pressure-assisted mode of operation. In another embodiment of the invention, pressurized air is supplied through a fluid damping system so as to minimize pressure spikes and consequent splatter when initiating a pressure-assisted mode of operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation-in-part of commonly-owned, copending U.S.patent application Ser. No. 08/687,790 filed Jul. 19, 1996.

FIELD OF THE INVENTION

[0002] This invention relates to applying a liquid to a substrate eitheras an atomized or nonatomized spray pattern. More particularly, theinvention relates to an improved method and apparatus for selectivelyapplying a coating of material on desired regions of a substrate such asa circuit board while avoiding the deposit of coating material onregions to be left uncoated.

BACKGROUND OF THE INVENTION

[0003] Typically, circuit boards that require protection from moisture,electric leakage and dust are coated with moisture proof insulativefilms, known as conformal coatings, such as, acrylic, polyurethane,silicone or epoxy synthetic resins dissolved in a volatile solvent. Whenapplied to clean circuit boards, an insulative resin film of uniformthickness and without pinholes, is formed as the solvent evaporates.

[0004] Spraying is the most commonly used insulative coating methodemployed in mass production. Spraying can be categorized as either airspraying in which an air stream is impinged against the stream of liquidcoating material after leaving the spray nozzle to form an atomizedspray pattern, or airless spraying in which the coating material isdispensed as a nonatomized spray pattern, as is disclosed in U.S. Pat.No. 5,294,459, assigned to Nordson Corp., which is hereby incorporatedby reference in its entirety.

[0005] In airless spraying of conformal coating material, as describedin the U.S. Pat. No. 5,294,459, the coating material is sprayed onto acircuit board as a flat, nonatomized pattern. Relative movement iseffected between the nozzle and the circuit board in a directiontransverse to the plane of the flat pattern discharged from the nozzle.The supply of coating material to the nozzle is intermittentlyinterrupted so as to prevent a deposit of liquid coating on regions ofthe circuit board and/or circuit components which are to be leftuncoated.

[0006] While air spraying, in which an air stream is impinged againstthe stream of liquid coating material to atomize the material afterleaving the spray nozzle, as disclosed in the U.S. Pat. No. 5,294,459,has proven to be an effective means of applying the material to asubstrate, there is still some overspray which increases the productioncosts of the circuit boards. Even with airless spraying, the distance ofthe nozzle to the circuit board sometimes causes the coating material tocoat regions which are to be left uncoated.

OBJECTS AND SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide apparatus andmethods for coating substrates, such as circuit boards, with either abead or an atomized conical, or a swirling spray pattern of liquidcoating material discharged from a nozzle mounted to a liquid spraydevice to obviate the problems and limitations of the prior art systems.

[0008] It is a further object of the present invention to provide anapparatus and method having three modes of operation for selectivelycoating substrates, such as circuit boards, with either a bead of liquidcoating material dispensed through an elongated nozzle tip or with anatomized, conical spray pattern formed by directing jets of relativelyhigh pressurized air against the bead of liquid coating materialdispensed through the elongated nozzle tip, or with a non-atomized,soft-swirling spray pattern formed by directing jets of relatively lowpressurized air against the bead of liquid coating material dispensedthrough the nozzle.

[0009] It is another object of the present invention to direct a portionof the jets of pressurized air against the bead of liquid coatingmaterial dispensed through the elongated nozzle tip and a portion of thepressurized air against the nozzle tip to stabilize the nozzle tip.

[0010] It is another object of the present invention to minimize thevolume of residual coating material present in the throughbore,consequently minimizing or substantially eliminating drip and dribbleduring startup.

[0011] It is another object of the present invention to minimizepressure spikes when initiating a pressure-assisted mode of operation,consequently minimizing or substantially eliminating splatter duringstartup.

[0012] In accordance with a first embodiment of the invention, a systemand method for spray coating a substrate with a liquid coating materialincludes a liquid spray device having a liquid delivery passageway forsupplying liquid coating material and an air delivery passageway forsupplying pressurized air. A nozzle mounted to the liquid spray devicehas an elongated nozzle tip extending outwardly therefrom and athroughbore in flow communication with the liquid delivery passageway. Anozzle orifice disposed at the end of the throughbore discharges theliquid coating material as a bead of liquid coating material. The nozzleincludes a plurality of bores in flow communication with the airdelivery passage. Each of the bores is formed at an angle with respectto the throughbore to direct pressurized air against both the elongatednozzle tip and the outer periphery of the bead of liquid coatingmaterial discharged from the elongated nozzle tip to form the liquidcoating material into an atomized, conical swirling spray pattern. Avalve controller is connected to the liquid spray device forintermittently opening and closing the air delivery passageway of theliquid spray device whereby the bead of liquid coating materialdischarged from the nozzle orifice is applied to the substrate either asthe atomized, conical, swirling spray pattern or as a bead of the liquidcoating material.

[0013] In accordance with a second embodiment of the invention, thesystem and method of the first embodiment can be used with a nozzleconstructed without the elongated nozzle tip.

[0014] In accordance with a third embodiment of the invention,pressurized air supplied to the spray nozzle is diverted to an exhaustport when operating the spray gun in a non-pressure-assisted mode ofoperation.

[0015] In accordance with a fourth embodiment of the invention,pressurized air supplied to the spray nozzle is reacted by a fluiddamping system when initiating operation of the spray gun in apressure-assisted mode of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The structure, operation, and advantages of the presentlypreferred embodiment of the invention will become further apparent uponconsideration of the following description taken in conjunction with theaccompanying drawings, wherein:

[0017]FIG. 1 is a side view, partly in cross section, of a coatingsystem having an elongated nozzle with a plurality of air bores disposedthereabout in accordance with the present invention;

[0018]FIG. 2 is a top view of the nozzle with air bores;

[0019]FIG. 3 is a view, in cross section, through line 3-3 of FIG. 2;

[0020]FIG. 4 is a top view of a circuit board having a band of coatingmaterial applied as an atomized conical spray pattern when the coatingsystem is operating in a first mode and a bead of coating materialapplied when the coating system is operating in a second mode;

[0021]FIG. 5 is a second embodiment of a spray nozzle adapted for usewith the coating system of the present invention;

[0022]FIG. 6 is an alternative design of a nozzle with six air bores;

[0023]FIG. 7A is an illustration of a pattern of coating materialapplied onto a substrate in a first (atomizing) mode of operation,according to the invention;

[0024]FIG. 7B is an illustration of a pattern of coating pattern appliedonto a substrate in a third (soft swirl) mode of operation, inaccordance with the invention;

[0025]FIG. 7C is an illustration of a pattern of coating pattern appliedonto a substrate in the third (soft swirl) mode of operation, inaccordance with the invention;

[0026]FIG. 8 is a cross-sectional view of a portion of an embodiment ofa coating system, according to the invention;

[0027]FIG. 9 is a graph illustrating the buildup of air pressure whenoperating the spray gun of the present invention;

[0028]FIG. 9A is a schematic illustration of an embodiment of a softstart aspect of the invention;

[0029]FIG. 9B is a schematic illustration of another embodiment of asoft start aspect of the invention; and

[0030]FIG. 9C is a graph illustrating the buildup of air pressure whenoperating the spray gun of the present invention using the soft starttechniques of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] Referring to FIG. 1, a coating system 10 for selectively coatinga substrate 11, typically a circuit board, with a moisture proof,insulative material is illustrated. The coating system 10 includes aspray gun (not shown) having a cylindrical extension (barrel) 12 with anozzle assembly 14 attached thereto. The overall construction of thecoating system 10 including a spray gun, a source of air and a source ofcoating material forms no part of this invention per se and is describedonly briefly herein. The nozzle assembly 14 could be attached, forexample, to the threaded end of extension of a spray gun, such as thespray gun disclosed in U.S. Pat. No. 5,294,459, which is herebyincorporated by reference in its entirety. The spray gun is operated bya conventional robot which controls the movement of the spray gun in thex, y and z axes.

[0032] The cylindrical extension 12 has a throughbore 16 housing areciprocating valve shaft 18 therein. Throughbore 16 is connected to asource (not shown) of pressurized, coating material 20 (see FIG. 2). Thecoating material includes a wide range of materials having a viscosityrange of about 10 cps (centipoise) to about 1000 cps. The invention ismost advantageously used with conformal material which is either solventfree or has a low percentage of solvents. However, it is within theterms of the invention to use the invention with conformal coatingmaterial with any typical percentage of solvents, generally known in theindustry. It is further within the scope of the invention to spray othertypes of liquid or viscous materials, as desired.

[0033] The coating material 20 is introduced into throughbore 16 at oneend of cylindrical extension 12 and flows along the outside of valveshaft 18 and through the outlet end 22 of extension 12. The lower end ofvalve shaft 18 is preferably tapered and formed to engage a valve seat24 located in a counterbore 26 formed at the lower end of throughbore16, adjacent outlet end 22 of cylindrical extension 12. The valve shaft18 reciprocates between an open, retracted open position, as shown inFIG. 1, in which its lower end disengages from valve seat 24 and opens adischarge outlet 28 therein, and an extended closed position (not shown)in which the lower end of valve shaft 18 engages seat 24 and blocks thedischarge outlet 28. The opening and closing of discharge outlet 28controls the discharge of coating material from throughbore 16 and intonozzle assembly 14 for deposition onto a substrate 11, such as a circuitboard.

[0034] Referring to FIG. 1, the nozzle assembly 14 includes a nozzlesupport 32 has a stepped throughbore 34 centered about a verticalcenterline 36. Throughbore 34 has an upper, threaded bore section 38which mates with the external threads on the lower end of extension 12.Throughbore 34 has a second bore section 40, below threaded bore section38 and adjoined thereto by an outwardly sloped frustro-conical surface42. Second bore section 40, as illustrated in FIG. 1, is adapted tocarry a seal 44, such as an O-ring, which engages seat 24 and forms afluid tight seal between nozzle assembly 14 and cylindrical extension12. Throughbore 34 has a third bore section 46, between second boresection 40 and a counterbore 48 disposed adjacent the outlet end 50 ofnozzle support 32.

[0035] The nozzle support 32 has an air inlet bore 52 disposed on oneside of throughbore 34. Inlet bore 52 has an inlet section 54 connectedto an air line 56. Air line 56 is connected to the outlet port 64 of acontroller, such as a solenoid actuated valve 66. Valve 66 is connectedby an air line 68 to a source of pressurized gas, typically air. Openingand closing valve 66 by an external control (not shown), controls theair jets impinging against the coating material being discharged fromnozzle support 32. Inlet bore 52 has an outlet section 72 connected to aring shaped outlet port opening 74 at the outlet end 50 of nozzlesupport 32.

[0036] Nozzle assembly 14 includes a spray nozzle 76 disposed at theoutlet end 50 of the nozzle support 32 and removably secured theretowith a nozzle nut 77 that is threadably secured to the threaded lowerend of nozzle assembly 14, as shown in FIG. 1. Referring to FIGS. 1, 2and 3, the spray nozzle 76 is shown in detail. Spray nozzle 76 is anannular plate 78 having one side formed with a first or upper surface 80and an opposite side formed with a second or lower surface 82 spacedfrom the upper surface 80. A boss 84 extends outwardly from the uppersurface 80, and an nozzle head 86 extends outwardly from the lowersurface 82 concentric to boss 84. A throughbore 88 is formed in spraynozzle 76 between the boss 84 and nozzle head 86 which has a dischargeoutlet 90. Spray nozzle 76 is mounted against the outlet end 50 ofnozzle support 32 so that boss 84 extends into counterbore 48 ofthroughbore 34. The inlet end 92 of throughbore 88 is formed with aradially inwardly tapering sidewall relative to the longitudinal axis 94of throughbore 88 and has a generally frustro-conical shape. An annular,V-shaped groove 96 is formed in the upper surface 80 of spray nozzle 76and extends inwardly toward the lower surface 82. Groove 96 is in flowcommunication with outlet port 74. As shown in FIGS. 2 and 3, twelve airjet bores 100 with a longitudinal axis 102 are formed in annular plate78. Bores 100 have an inlet opening 103 intersecting annular groove 96and an outlet opening 105 intersecting lower surface 82 at 30 degreeintervals there along. In the preferred embodiment, the bores 100 have adiameter from about 0.013 inches (13 mils) to about 0.024 inches (24mils). Preferably, as illustrated in FIG. 3, the longitudinal axis 102through each air jet bore 100 is formed at an angle “a” of approximately30 degrees with respect to the longitudinal axis 94 of the throughbore88. However, it is within the terms of the invention to change the angle“a” to accommodate nozzle tips 86 of different sizes. Also, while theillustrated embodiment is described with twelve, equally spaced jetbores 100, it is within the terms of the invention to use six or moreequally spaced jet bores 100, as shown in FIG. 6.

[0037] In one embodiment of the invention, nozzle head 86 has anelongated nozzle tip 87 constructed of a tube 104 having an inlet end106 and an outlet end 108, and is mounted in throughbore 88 and securedtherein by conventional means such as brazing. Inlet end 106 of tube 104preferably extends to the intersection of inlet end 92 and throughbore88. Outlet end 108 extends outward from the discharge outlet 90 ofnozzle head 86 and terminates in a nozzle orifice 107. In the preferredembodiment, tube 104 extends a distance of about 0.250 inches to about1.0 inches from discharge outlet 90. A bore extending through tube 104of about 0.008 inches to about 0.050 inches in diameter terminates witha nozzle orifice 107. While tube 104 is shown as being mounted withinbore 88 of spray nozzle 76, it is also within the terms of the inventionto manufacture nozzle 76 with tube 104 as a unitary element.

[0038] As shown in FIG. 1, the longitudinal axis 102 through each of theair jet bores 100 is angled to intersect both a portion of tube 104projecting outward from discharge end 90 as well as a bead 112 ofcoating material 20 discharged from the outlet end 108 of the tube. Theair flow from jet bores 100 is directed against tube 104 acts tostabilize the tube and reduce any vibration thereof. As discussed indetail below, the jets of air passing through each of the equally spacedair jet bores 100 impact the outside of the bead 112 of coating material20 to atomize coating material 20 and to cause the atomized coatingmaterial to form a conical spray pattern for deposition onto substrate11. In addition, the jets of air cause the atomized coating material toswirl within the conical spray pattern. Depending on the length of thesection of tube 104 which projects from outlet 90, the angle of axis 102with respect to axis 94 extending through bore 88 can be increased ordecreased to control the proportion of air flow impingement againstelongated nozzle tip 87 and against bead 112. Also, the spacing of theoutlets 105 of jet bores 100 can be spaced closer or further from axis94 through throughbore 88 as required to further control the proportionof air flow impingement against elongated nozzle tip 87 and against bead112.

[0039] A flow path for coating material 20 extends from cylindricalextension 12 to the nozzle assembly 14 which includes throughbore 16 inextension 12, the discharge outlet 28 of valve seat 24, the throughbore34 in nozzle assembly 14 and the bore through elongated nozzle tip 87terminating in orifice 107, respectively. The flow of coating materialthrough this flow path is controlled by valve shaft 18 moving between aretracted, open position relative to discharge outlet 28 in valve seat24, and a closed, extended position relative to discharge outlet 28.

[0040] The system 10, typically used for spraying a circuit board 11with coating material 20, has two modes of operation for applying thecoating material to selective areas of the circuit board withoutapplying the coating material to regions that are to be left uncoated.In a first mode of the present invention when the air jets are directedagainst the bead of coating material being discharged from the nozzleorifice 107 of elongated nozzle tip 87, a high quality, pin hole free,thin coating of material between about 0.5 mils (0.0005 inches) andabout 5 mils (0.005 inches) can be applied to substrate 11. In a secondmode of the invention when the air jets are turned off, a bead ofcoating of material between about 5 mils and about 10 mils can beapplied to substrate 11.

[0041] In the first mode of operation, valve 66 opens to direct jets ofair through air bores 100. Next, the flow of liquid coating material isinitiated by moving valve shaft 18 from an extended, closed positionrelative to discharge outlet 28 in valve seat 24, to an open, retractedposition relative to discharge outlet 28. The liquid coating materialnow flows from a source (not shown) into throughbore 16 in extension 12,through discharge outlet 28 of valve seat 24, through throughbore 34 innozzle assembly 14, through tube 104 and exits from nozzle orifice 107of elongated nozzle tip 87 as a bead 112 of coating material. Thepressure of the liquid coating material delivered to cylindricalextension 12, in the first mode, is about 5 psi (pounds per square inch)to about 60 psi and preferably between about 10 psi and 30 psi. The jetsof air impinge substantially tangential to the outer periphery of bead112 of coating material discharged from nozzle orifice 107 of elongatednozzle tip 87 to break the liquid coating material into droplets andform an atomized spray having a generally conical shape, as illustratedin FIG. 1. In the first mode of operation, the air is delivered at apressure of about 10 psi to about 70 psi and preferably about 10 psi toabout 20 psi. The atomized spray has a tendency to swirl within theconical shape because of the direction with which the bead of liquidmaterial discharged from the nozzle is impinged from opposing sides byjets of pressurized air from air bores 100 and thereby forms a densercoating on the board. Typically, during application of the coatingmaterial 20, the board 11 is held in a stationary position and the robotto which the spray gun is mounted moves the gun in a preprogrammedpattern. A particular advantage of the present invention is that therobot can move in the z axis so that the end 108 of tube 104 ispositioned very close to the circuit board, i.e. about 0.250 inches toabout 1.0 inches from substrate 11, so that the atomized conical spraypattern is tightly controlled and can be applied near the edges of thecircuit board 11 or close to a circuit component 109 mounted to theupper surface 110 of the board. While the first mode of operation iseffective for general spraying of circuit boards, a small, butnoticeable overspray and splatter is sometimes produced by the conicalspray pattern and prevents the use of air impingement when spraying thecoating material along the edge of the board or directly adjacent to aregion (typically having a circuit component such as a switch) which isto be left uncoated.

[0042] Under the latter circumstances, the second mode of operation isemployed wherein the air valve 66 is closed and the coating material isapplied as a nonatomized bead 112. In the second mode of operation, thecoating material is delivered to cylindrical extension 12 at a pressureof about 30 psi to about 60 psi. A particular advantage of the presentinvention is the ability of the robot to move the end 108 of tube 104very close to the circuit board 11. For example, when a circuit element109 projects upward from the upper surface 110 of circuit board 11, asshown in FIG. 4, tube 104 can be positioned very close to the board sothat the bead 112 of coating material is applied directly adjacent thecircuit element without any contact with the coating material. Then, therobot can raise the end 108 of tube 104 away from surface 110 so thatthe tube can move across element 109 without any interference therewith.In selected applications, the bead 112 can also be applied along theedges of circuit board 11.

[0043] It can be appreciated by one skilled in the art that the two modesystem of the present invention is adapted for use with a spray gunwhich can be automatically controlled by a robot. When an area of acircuit board requires an insulative coating, a first mode is preferablyoperated to apply an atomized coating of insulative material preferablyhaving a thickness of about 0.5 mils to about 5 mils. However, wheneverthe area being sprayed is directly next to a region which is to be leftuncoated, the system is switched into the second mode whereby anonatomized bead of coating material is applied. While the coating fromthe second mode is thicker, i.e. about 5 mils to about 10 mils, thanotherwise desired, it is typically only applied to a small, confinedarea and the overall coating process is economical.

[0044] In a second embodiment of the invention, the spray nozzle 76 isreplaced with a spray nozzle 120, as shown in FIG. 5, which is identicalto nozzle 76 except for the omission of elongated nozzle tip 87 fromnozzle head 86. The coating system 10 operates with spray nozzle 120 asdescribed in the first embodiment except for the coating materialflowing through throughbore 88′ and discharging from discharge outlet90′ of nozzle head 86′. As with the first embodiment, air jets aredirected through air bores 100′ substantially tangential to the outerperiphery of the bead of coating material discharged from nozzle head86′. Throughout the specification primed numbers represent structuralelements which are substantially identical to structural elementsrepresented by the same unprimed number.

[0045] While the present invention has been described with reference tousing jets of air, it is also within the terms of the invention to useany type of desired gas.

[0046] Soft Swirl Mode of Operation

[0047] As discussed hereinabove, the coating system 10 is operable intwo modes of operation, a first mode dispensing a conical spray ofatomized coating material and a second mode dispensing a bead ofnonatomized coating material to selectively dispense a conformal (e.g.,protective) coating on substrates such as circuit boards.

[0048] When applying coating materials to a substrate such as a circuitboard, it is important that the transfer efficiency be as high aspossible, and there are challenges associated with applying coatingmaterial adjacent components on the circuit board.

[0049] It is thus an object of the invention to provide the coatingsystem 10 with another, third mode of operation.

[0050] According to a feature of the invention, the coating system isoperable in an additional third mode of operation, and can be switchedbetween any of the three modes of operation on the fly (while coating asingle substrate).

[0051]FIG. 7A illustrates a pattern of coating material generated on asubstrate in the first (atomizing) mode, discussed hereinabove. As thespray gun (not shown) is moved linearly along the surface of thesubstrate, a completely filled-in circular pattern of coating materialis applied from a first position 130 (shown in solid lines) to a secondposition 130′ (shown in dashed lines) displaced from the first position.As discussed above, the first (atomizing) mode is suitably initiated byat least 10 psi of air pressure.

[0052] In the second (bead) mode, a bead of coating material is laiddown on the substrate while the spray gun is moved linearly along thesurface of the substrate.

[0053] In the third (soft swirl) mode of operation, air pressure in therange of from 4 psi to 10 psi is provided, which is sufficient to directthe coating material to a sequence of selected positions on thesubstrate, but not to atomize it.

[0054]FIG. 7B illustrates the coating pattern that will result from thespray gun being operated in the third (soft swirl) mode when the spraynozzle is not moving with respect to the substrate. This results in acoating pattern of a small, filled-in circle 132 of coating materialwhich moves around the centerline 36 of the spray nozzle 76. The path ofthe coating material is shown in dashed lines. This would result in adonut-shaped area of coating coverage on the substrate, with a voidarea.

[0055]FIG. 7C illustrates the result that would accrue in the third(soft swirl) mode of operation when the spray gun is moving linearlyalong the surface of the substrate, from a first position 134 (shown insolid lines) to a 134′ second position (shown in dashed lines) displacedfrom the first position. Depending on the speed that the spray gun ismoving along the surface of the substrate, this will result in asinusoidal or overlapping loop (soft swirl) pattern of coating materialon the substrate. Such a soft swirl application of coating material(third mode) exhibits 100% transfer efficiency, without atomizing thecoating material, and is generally preferred when applying coatingmaterial adjacent components on the circuit board substrate. The patternof coating material deposited onto the substrate in the third (softswirl) mode of operation is comparable to the spiral patterns ofadhesive created by the nozzles in commonly-owned U.S. Pat. No.4,969,602 issued Nov. 13, 1990, and in commonly-owned U.S. Pat. No.5,194,115 issued Mar. 16, 1993.

[0056] There has thus been described a coating system with three modesof operation: a first pressure-assisted (at least 10 psi) mode whereincoating material is applied as an atomized conical spray pattern ontothe substrate; a second non-pressurized mode wherein a bead of coatingmaterial is applied onto the substrate; and a third pressure-assisted(at least 4 and less than 10 psi) mode wherein coating material isapplied in a soft swirl pattern onto the substrate without beingatomized. These three modes may selectively be initiated and employedduring the coating of a single substrate to deposit highly conformalcoatings on selected areas of the substrate, without waste, therebyminimizing on the expense associated with the coating material.Different areas of the substrate can be coated with the spray gunoperating in any one of the three modes.

[0057] Slim Swirl

[0058] In the coating system 10 described hereinabove, the valve shaft18 reciprocates between an open, retracted position, as shown in FIG. 1,in which its lower end disengages from valve seat 24, and an extendedclosed position in which the lower end of the valve shaft 18 engagesseat 24 and blocks the discharge outlet 28.

[0059] When the discharge outlet 28 is closed off, there is a residualamount of coating material disposed in the volume of the throughbore 34.It has been observed that, when the discharge outlet 28 is subsequentlyopened (i.e., when the valve shaft 18 disengaged from the valve seat24), the spray nozzle 76 will drip and dribble coating material uponstartup.

[0060] It is thus an object of this invention to minimize such drip anddribble upon startup.

[0061] According to a feature of the invention, the spray nozzle isdisposed in close proximity to the valve seat, thereby minimizing thevolume of residual coating material present in the throughbore, andconsequently minimizing or substantially eliminating drip and dribbleduring startup.

[0062]FIG. 8 illustrates a relevant portion of a coating system 140,similar to the coating system 10. Certain common elements between thecoating system 140 and the coating system 10 are referred to by theirnumbers (in parentheses) associated with the coating system 10, forillustrative clarity.

[0063] In this embodiment, the spray nozzle 142 is disposed at the endof and is an integral part of the barrel 144 (compare 12). The spraynozzle 142 has air jet bores (100) terminating in outlet openings (105),and has an inlet end (92), a throughbore (88) and a discharge outlet(90), and may be provided with a tube (104) in the manner describedhereinabove with respect to the spray nozzles 76 and 87. For purposes ofthe following discussion, the spray nozzle 142 is identical to the spraynozzle 76.

[0064] In this embodiment of a coating system 140, a cylindrical member146 is disposed within the barrel 144, and the valve shaft 148 (compare18) is disposed within the cylindrical member 146. The cylindricalmember 146 has a throughbore 150 (compare 16) which functions as a fluiddelivery passageway. The valve seat 152 (compare 24) is disposed at theend of the cylindrical member 146.

[0065] The valve shaft 148, cylindrical member 146, barrel 144, valveseat 152 and spray nozzle 142 are all concentric about a verticalcenterline 156 (compare 36).

[0066] In use, the valve shaft 148 reciprocates between an open,retracted position, as shown in FIG. 8, in which its lower enddisengages from valve seat 152 and opens a discharge outlet 154 (compare28) therein, and an extended closed position (not shown) in which thelower end of valve shaft 148 engages the valve seat 152 and blocks thedischarge outlet 154. The opening and closing of the discharge outlet154 controls the flow of coating material through the liquid deliverypassageway 150 to the spray nozzle 142 for deposition onto a substrate11, such as a circuit board. In this embodiment, a ball 158 is affixed,such as by brazing, to the lower end of the valve shaft 128 and seals inthe extended closed position of the valve shaft 148 against thedischarge outlet 154 of the valve seat 152.

[0067] The overall construction of the coating system 140 including aspray gun, a source of air and a source of coating material forms nopart of this invention per se and is described only briefly herein. Thespray gun is operated by a conventional robot which controls themovement of the spray gun in the x, y and z axes.

[0068] In this embodiment of a coating system 140, the spray nozzle 142is disposed closely adjacent the valve seat 152, thereby essentiallyeliminating residual coating material accumulating in a volume betweenthe valve seat 152 and the spray nozzle 142. The spray nozzle 142 has aboss (84) on its upper surface (80) which fits into a correspondingcounterbore (48) on the lower surface of the valve seat 152. To ensure agood seal between the spray nozzle 142 and the valve seat 152, a washer160 is disposed between the upper surface of the spray nozzle 142 andthe lower surface of the valve seat 152. The washer 160 has an innerdiameter sufficiently large to fit around the boss on the upper surfaceof the spray nozzle 142 and an outer diameter which is sufficientlysmall so as not to block the inlet openings (103) of the air jet bores(100).

[0069] In this embodiment of a coating system 140, an air deliverypassageway 162 (compare 52) is formed between the outer surface of thecylindrical member 146 and the inner surface of the barrel 144. An upperend of the air passage 162 is sealed off. A lower end of the airdelivery passageway 162 is in flow communication with the air jet bores(100) in the spray nozzle 142. Pressurized air is provided to the airpassage from any suitable source (not shown), such as has been describedhereinabove, through an air inlet 164 extending through the barrel 144into the air passage 162, so that the coating system 140 may operate inany of the three modes of operation described herein.

[0070] Eliminating Pressure Transients

[0071] Coating systems operating in various modes of operation,including selectively providing pressurized air (in the first and thirdmodes of operation) and not providing pressurized air (in the secondmode of operation) have been described hereinabove. It has been observedthat upon initiating a mode which includes providing air pressure, thatthere is a startup pressure spike which may be up to approximately six(6) times the desired flow rate. This can result in undesirable splatterof coating material on the substrate.

[0072]FIG. 9 is a graph illustrating the aforementioned problem ofpressure spikes occurring when initiating a mode which includesproviding air pressure. The x-axis is time, and the y-axis is pressure).This graph 170 illustrates that it is desired to provide the spray gunwith 25 units (e.g., 25 psi) of air pressure, commencing at 10milliseconds (10 ms) from an initial pressure of 0 psi at t0. As isillustrated by the line 172, a pressure spike commences at 10 ms andcontinues upward in excess of 125 psi in the interval between 10 ms andapproximately 27 ms. Then, between 27 ms and approximately 35 ms thepressure settles down to the desired 25 psi flow rate to operate thespray gun in a pressure-assisted mode (e.g., in the first atomizingmode). It is thus evident that there is a startup pressure spike,lasting 15 ms from startup, which may be up to approximately six (6)times the desired flow rate. This can result in undesirable splatter ofcoating material on the substrate. What is needed is a technique forproviding a soft start upon initiating a pressure-assisted mode ofoperation.

[0073] It is therefore an object of the invention to substantiallyreduce or eliminate pressure spikes attendant initiating apressure-assisted mode.

[0074] According to an embodiment of the invention, the airflow(pressure) being directed to the spray gun is directed by a valve to anexhaust port in a noncoating mode (or in the second, non-pressurized,non-atomizing, bead coating mode) so that a static-to-dynamic pressurechange does not occur upon initiating dispensing the coating material orinitiating a pressure-assisted mode of operation (i.e., the firstatomizing or third soft swirl operating modes described hereinabove).When the pressure-assisted mode(s) is (are) enabled, a valve simplyredirects the airflow to the coating gun rather than to the exhaustport.

[0075]FIG. 9A illustrates an embodiment 174 of a soft start technique ofthe invention, wherein the airflow (pressure) being directed to thespray gun is directed by a valve 176 to an exhaust port over a line 178in a noncoating mode (or in the second, non-pressurized, non-atomizing,bead coating mode) so that a static-to-dynamic pressure change does notoccur upon initiating a pressure-assisted mode of operation. When thepressure-assisted mode(s) is (are) enabled, the valve 176 simplyredirects the airflow through an outlet port 64 (see FIG. 1) to thecoating gun, over the line 56 (see FIG. 1) rather than to the exhaustport 178. The valve 176 is suitably a solenoid actuated valve and isconnected by an air line 68 to a source of pressurized gas, typicallyair. Opening and closing the valve 176 by an external control (notshown), controls the air jets impinging against the coating materialbeing discharged from the nozzle (76, 142).

[0076]FIG. 9B illustrates another embodiment 180 of a soft starttechnique for eliminating pressure spikes when switching on airflow froma supply 182 of pressurized gas, typically air, to the spray gun over aline 56. Generally, a fluid damping system is provided having thefollowing major components and construction.

[0077] A piston 184 is disposed within a cylindrical chamber 186. Aspring 188 is disposed atop the piston 184 and a rod 190 extends fromthe lower surface of the piston 184. The lower (as viewed) end of therod 190 is suitably shaped (e.g., tapered) to seal against an inlet sideof an orifice in a valve seat 192. The rod 190 is illustrated in its“closed” position, against the valve seat 192, and is biased to theclosed position by the spring 188 exerting a downward (as viewed)closing force on the piston 184. The line 56 is connected to an outletside of the valve seat 192.

[0078] Pressurized air from the source 182 of pressurized gas isregulated by an air regulator 194, and is provided over a line 196 to achamber 198 adjacent the valve seat. In this manner, when the rod 190 isin its “open” position (not shown), regulated pressurized air from thesource 182 of pressurized gas can flow through the line 56 to the spraygun.

[0079] The rod 190 is shown in its closed position, and is moved upwardto its open position in any suitable manner such as by providingsufficient air pressure to the bottom side of the piston 184 to overcomethe closing force of the spring 188. To this end, a solenoid valve 200receives unregulated (prior to being regulated by the regulator 194) airpressure from the source 182 of pressurized gas, and an outlet of thesolenoid valve 200 is connected via a line 202 to a portion of thecylindrical chamber 186 which is underneath the piston 184. The solenoidvalve 200 is switched on (opened) and off (closed) in a manner similarto that in which the opening and closing of the valve 66 was controlled(i.e., by an external control, not shown). The upward stroke of thepiston 184 is limited by a set screw 204, such as a thumb screw)entering the top of the cylindrical chamber 186 and acting against thetop surface of the piston 184.

[0080] Movement of the rod 190 from its closed position to its openposition is dampened, in the following manner. The portion of thecylindrical chamber 186 atop the piston 184 is filled with fluid (notshown), such as hydraulic oil which is supplied from an oil reservoir206. The oil reservoir 206 is connected to the chamber 186 via a line208 to a flow control valve (orifice) 210, and via a line 212 from theflow control valve 210 to the chamber 186. Damping is effected by theflow control valve 210 restricting the free flow of oil back and forthbetween the chamber 186 and the reservoir 206. The flow control valve210 is adjustable, to control the amount of damping exerted on themovement of the piston 184.

[0081] In this manner, when a pressure-assisted mode is enabled,unregulated air pressure moves the piston 184 and rod 190 to an openposition, regulated air pressure is provided over the line 56 to thespray gun, and pressure transients are absorbed by the damping mechanismof the oil moving through the flow control valve 210 as the piston 184moves upward, so that the aforementioned pressure transients andsplatter are avoided when operating the spray gun in a pressure-assistedmode.

[0082]FIG. 9C is a graph illustrating the results achieved by employingthe soft start techniques of the embodiments 174 or 180 wherein theaforementioned problem of pressure spikes occurring when initiating amode which includes providing air pressure is overcome. As with FIG. 9,the x-axis is time, and the y-axis is pressure). This graph 210 (compare170) illustrates that it is desired to provide the spray gun with 25units (e.g., 25 psi) of air pressure, commencing at 10 milliseconds (10ms) from an initial pressure of 0 psi at t₀. As is illustrated by theline 212 (compare 172), in the interval between 10 ms and approximately18 ms, the pressure rises smoothly to the desired exemplary 25 psi flowrate to operate the spray gun in a pressure-assisted mode (e.g., in thefirst atomizing mode). It is thus evident that the problem of incurringa startup pressure spike, and attendant splatter of coating materialupon startup, has substantially been reduced, if not entirelyeliminated.

[0083] It is apparent that there has been provided in accordance withthis invention apparatus and methods for coating selected areas of acircuit board with a liquid coating material without coating regions ofthe circuit board to be left uncoated that satisfy the objects, meansand advantages set forth hereinbefore. In one mode of operation of afirst embodiment of the invention, a bead of liquid coating materialdispensed from an elongated nozzle tip is atomized by directing jets ofair against the bead to form a conical, swirling spray pattern which issprayed on selected areas of a circuit board where overspray is not asignificant concern. In a second mode, the non-atomized bead of liquidcoating material dispensed through the elongated nozzle tip is appliedon selected areas of the circuit board where overspray cannot betolerated. In a second embodiment of the invention, a first modeoperates by dispensing the bead of liquid coating material from a nozzlewithout the elongated nozzle tip and atomizing the bead of material bydirecting jets of air against the bead to form a conical, swirling spraypattern which is sprayed on selected areas of a circuit board whereoverspray is not a significant concern. In a second mode of the secondembodiment, the bead of liquid coating material dispensed from thenozzle without the elongated nozzle tip is applied on selected areas ofthe circuit board where overspray cannot be tolerated.

[0084] While the invention has been described in combination withembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, the invention isintended to embrace all such alternatives, modifications and variationsas fall within the spirit and broad scope of the appended claims.

What is claimed is:
 1. A system for spray coating a substrate with aliquid coating material, comprising: a liquid spray device having aliquid delivery passageway for supplying liquid coating material and anair delivery passageway for supplying pressurized air; a nozzle mountedto said liquid spray device having an elongated nozzle tip extendingoutwardly therefrom and having a throughbore in flow communication withsaid liquid delivery passageway with a nozzle orifice disposed at theend of said throughbore to discharge said liquid coating material as abead of liquid coating material, said nozzle including a plurality ofair bores in flow communication with said air delivery passage to directpressurized air against the outer periphery of said bead of liquidcoating material discharged from said elongated nozzle tip to form saidliquid coating material into an atomized spray pattern; and a controlleroperatively connected to said liquid spray device for intermittentlyopening and closing said air delivery passageway of said liquid spraydevice whereby said bead of liquid coating material discharged from saidnozzle orifice is applied to said substrate either as an atomized, spraypattern or as a bead of said liquid.
 2. The system of claim 1 whereinsaid nozzle is formed with at least six bores in flow communication withsaid air delivery passage, said air bores being formed at an angle withrespect to said throughbore.
 3. The system of claim 2 wherein saidnozzle is formed with at least twelve air bores in flow communicationwith said air delivery passage.
 4. The system of claim 2 wherein saidelongated nozzle tip includes a tube extending outward from said nozzle,said tube having said nozzle orifice disposed at one end to dischargesaid liquid coating material as a bead of liquid coating material. 5.The system of claim 4 wherein said nozzle comprises: an annular platewith a first surface on one side of said plate, and a second surface ona second opposite side of said plate having a nozzle head extendingoutwardly therefrom, said plate having a throughbore extending betweensaid one side and said nozzle head to receive said tube; and said platebeing formed with said air bores extending between said first surfaceand said second surface.
 6. The system of claim 4 wherein said liquidspray device has a valve shaft movable between an retracted openposition in which the lower end of said valve shaft is spaced from avalve seat and uncovers a discharge outlet therein, and an extendedclosed position in which said lower end engages said valve seat andblocks said discharge outlet to control the discharge of coatingmaterial from said liquid spray device.
 7. The system of claim 6 whereinsaid controller is a solenoid valve.
 8. A method of applying a liquidcoating material to a substrate, comprising the steps of: supplyingliquid coating material through a liquid delivery passageway of a liquidspray device and supplying pressurized air to an air delivery passagewayof said liquid spray device; directing said liquid coating material fromsaid liquid delivery passageway into a throughbore extending through anozzle mounted to said spray device and through an elongated nozzle tipextending outwardly from said nozzle, said throughbore terminating in anozzle orifice for discharging said liquid coating material from saidnozzle orifice as a bead of liquid coating material; directing saidpressurized air from said air delivery passageway into a plurality ofair bores formed in said nozzle so that pressurized air flowing throughsaid air bores is directed against the outer periphery of said bead ofliquid coating material to form a spray pattern of atomized liquidcoating material for deposition on a substrate; and intermittentlyopening and closing said air delivery passageway of said liquid spraydevice whereby said bead of liquid coating material being dischargedfrom said nozzle orifice is applied to said substrate either as anatomized spray pattern or as a bead.
 9. The method of claim 8 includingthe step of directing said pressurized air from said air deliverypassageway into at least six air bores formed in said nozzle at an anglewith respect to said throughbore.
 10. The method of claim 9 includingthe step of directing said pressurized air from said air deliverypassageway into twelve air bores formed in said nozzle at an angle withrespect to said throughbore.
 11. The method of claim 8 wherein saidliquid coating material is conformal coating material and said substrateis a circuit board.
 12. The method of claim 8 including the step ofdirecting said pressurized air against said outer periphery of said beadof liquid coating material to form a swirling, conical spray pattern ofatomized liquid coating material.
 13. The method of claim 8 includingthe step of directing a portion of said pressurized air against saidelongated nozzle tip to reduce vibration of said nozzle tip.
 14. Asystem for spray coating a substrate with a liquid coating material,comprising: a liquid spray device having a liquid delivery passagewayfor supplying liquid coating material and an air delivery passageway forsupplying pressurized air; a nozzle mounted to said liquid spray devicehaving a discharge outlet at one end and having a throughbore in flowcommunication with said liquid delivery passageway to discharge saidliquid coating material as a bead of liquid coating material from saiddischarge outlet, said nozzle including a plurality of air bores in flowcommunication with said air delivery passage to direct pressurized airagainst the outer periphery of said bead of liquid coating materialdischarged from said discharge outlet to form said liquid coatingmaterial into an atomized spray pattern; and a controller operativelyconnected to said liquid spray device for intermittently opening andclosing said air delivery passageway of said liquid spray device wherebysaid bead of liquid coating material discharged from said dischargeoutlet is applied to said substrate either as an atomized spray patternor as a bead of said liquid.
 15. The system of claim 14 wherein saidnozzle is formed with at least six air bores formed at an angle withrespect to said throughbore and in flow communication with said airdelivery passage.
 16. The system of claim 15 wherein said nozzle isformed with twelve air bores formed at an angle with respect to saidthroughbore and in flow communication with said air delivery passage.17. The system of claim 16 wherein said nozzle comprises: an annularplate with a first surface on one side of said plate, and a secondsurface on a second opposite side of said plate having a nozzle headextending outwardly therefrom, said plate having a throughbore extendingbetween said first surface and said nozzle head; and said plate beingformed with said plurality of air bores extending between said firstsurface and said second surface, each of said plurality of air boresbeing formed at an angle with respect to said throughbore in said plate.18. A method of applying a liquid coating material to a substrate,comprising the steps of: supplying liquid coating material through aliquid delivery passageway of a liquid spray device and supplyingpressurized air to an air delivery passageway of said liquid spraydevice; directing said liquid coating material from said liquid deliverypassageway into a throughbore extending through a nozzle and terminatingin a discharge outlet whereby said liquid coating material is dischargedfrom said discharge outlet as a bead of liquid coating material;directing said pressurized air from said air delivery passageway into aplurality of air bores formed in said nozzle so that pressurized airflowing through said air bores is directed against the outer peripheryof said bead of liquid coating material to form a spray pattern ofatomized liquid coating material for deposition on a substrate; andintermittently opening and closing said air delivery passageway wherebysaid bead of liquid coating material being discharged from saiddischarge outlet is applied to said substrate either as an atomizedspray pattern or as a bead.
 19. The method of claim 18 including thestep of directing said pressurized air from said air delivery passagewayinto at least six air bores formed in said nozzle at an angle withrespect to said throughbore.
 20. The method of claim 19 including thestep of directing said pressurized air from said air delivery passagewayinto twelve air bores formed in said nozzle at an angle with respect tosaid throughbore.
 21. The method of claim 18 wherein said liquid coatingmaterial is conformal coating material and said substrate is a circuitboard.
 22. The method of claim 18 including the step of directing saidpressurized air against said outer periphery of said bead of liquidcoating material to form a swirling, conical spray pattern of atomizedliquid coating material.
 23. A method of applying a liquid coatingmaterial to a substrate, comprising the steps of: supplying liquidcoating material through a liquid delivery passageway of a liquid spraydevice and supplying pressurized air to an air delivery passageway ofsaid liquid spray device; directing said liquid coating material fromsaid liquid delivery passageway into a throughbore extending through anozzle mounted to said spray device and through an elongated nozzle tipextending outwardly from said nozzle, said throughbore terminating in anozzle orifice for discharging said liquid coating material from saidnozzle orifice as a bead of liquid coating material; selectivelydirecting said pressurized air from said air delivery passageway into aplurality of air bores formed in said nozzle to operate the liquid spraydevice in three modes of operation; a first mode of operation whereinpressurized air flowing through said air bores at a first pressure isdirected against the outer periphery of said bead of liquid coatingmaterial to form a spray pattern of atomized liquid coating material fordeposition on a substrate; a second mode of operation wherein no air isflowed through said air bores and said liquid coating material inapplied as a bead; and a third mode of operation wherein pressurized airflowing through said air bores at a second pressure lower than the firstpressure is directed against the outer periphery of said bead of liquidcoating material to form a spray pattern of non-atomized liquid coatingmaterial for deposition on a substrate.
 24. The method of claim 23wherein the first pressure is greater than 10 psi and the secondpressure is between 4 psi and 10 psi.
 25. The method of claim 23including the step of directing said pressurized air from said airdelivery passageway into at least six air bores formed in said nozzle atan angle with respect to said throughbore, in the first and third modesof operation.
 26. The method of claim 25 including the step of directingsaid pressurized air from said air delivery passageway into twelve airbores formed in said nozzle at an angle with respect to saidthroughbore, in the first and third modes of operation.
 27. The methodof claim 23 wherein said liquid coating material is conformal coatingmaterial and said substrate is a circuit board.
 28. The method of claim23 including the step of directing said pressurized air against saidouter periphery of said bead of liquid coating material to form aswirling, conical spray pattern of atomized liquid coating material, inthe first and third modes of operation.
 29. The method of claim 23including the step of directing a portion of said pressurized airagainst said elongated nozzle tip to reduce vibration of said nozzletip, in the first and third modes of operation.
 30. A system for spraycoating a substrate with a liquid coating material, comprising: a liquidspray device having a liquid delivery passageway for supplying liquidcoating material and an air delivery passageway for supplyingpressurized air; a valve shaft movable between an retracted openposition in which the lower end of said valve shaft is spaced from avalve seat and uncovers a discharge outlet therein, and an extendedclosed position in which said lower end of the valve shaft engages saidvalve seat and blocks said discharge outlet to control the discharge ofcoating material from said liquid spray device; a nozzle mounted to saidliquid spray device in close proximity to the valve seat, said nozzlehaving an elongated nozzle tip extending outwardly therefrom and havinga throughbore in flow communication with said liquid delivery passagewaywith a nozzle orifice disposed at the end of said throughbore todischarge said liquid coating material as a bead of liquid coatingmaterial, said nozzle including a plurality of air bores in flowcommunication with said air delivery passageway to direct pressurizedair against the outer periphery of said bead of liquid coating materialdischarged from said elongated nozzle tip to form said liquid coatingmaterial into an atomized spray pattern; and a controller operativelyconnected to said liquid spray device for intermittently opening andclosing said air delivery passageway of said liquid spray device wherebysaid bead of liquid coating material discharged from said nozzle orificeis applied to said substrate either as an atomized spray pattern or as abead of said liquid.
 31. The system of claim 30 wherein said nozzle isformed with at least six air bores in flow communication with said airdelivery passage, said air bores being formed at an angle with respectto said throughbore.
 32. The system of claim 31 wherein said nozzle isformed with at least twelve air bores in flow communication with saidair delivery passage.
 33. The system of claim 31 wherein said elongatednozzle tip includes a tube extending outward from said nozzle, said tubehaving said nozzle orifice disposed at one end to discharge said liquidcoating material as a bead of liquid coating material.
 34. The system ofclaim 33 wherein said nozzle comprises: an annular plate with a firstsurface on one side of said plate, and a second surface on a secondopposite side of said plate having a nozzle head extending outwardlytherefrom, said plate having a throughbore extending between said oneside and said nozzle head to receive said tube; and said plate beingformed with said air bores extending between said first surface and saidsecond surface.
 35. The system of claim 34 wherein said controller is asolenoid valve.
 36. A method of applying a liquid coating material to asubstrate, comprising the steps of: supplying liquid coating materialthrough a liquid delivery passageway of a liquid spray device andsupplying pressurized air from a source of pressurized air to an airdelivery passageway of said liquid spray device; directing said liquidcoating material from said liquid delivery passageway into a throughboreextending through a nozzle mounted to said spray device and through anelongated nozzle tip extending outwardly from said nozzle, saidthroughbore terminating in a nozzle orifice for discharging said liquidcoating material from said nozzle orifice as a bead of liquid coatingmaterial; selectively directing said pressurized air from said airdelivery passageway into a plurality of air bores formed in said nozzleto operate the liquid spray device in at least two modes of operation;at least one pressure-assisted mode of operation wherein pressurized airflows through said air bores and is directed against the outer peripheryof said bead of liquid coating material; and a non-pressure-assistedmode of operation wherein no air is flowed through said air bores andsaid liquid coating material is applied as a bead; in each of the atleast two modes of operation, intermittently opening and closing saidair delivery passageway of said liquid spray device whereby said bead ofliquid coating material being discharged from said nozzle orifice isapplied to said substrate either as an atomized spray pattern, anon-atomized spray pattern or as a bead; further comprising: disposing avalve in a line between the supply of pressurized air and the airdelivery passageway of said liquid spray device; and in thenon-pressure-assisted mode of operation, controlling the valve to divertsaid pressurized air to an exhaust port.
 37. A method of applying aliquid coating material to a substrate, comprising the steps of:supplying liquid coating material through a liquid delivery passagewayof a liquid spray device and supplying pressurized air from a source ofpressurized air to an air delivery passageway of said liquid spraydevice; directing said liquid coating material from said liquid deliverypassageway into a throughbore extending through a nozzle mounted to saidspray device and through an elongated nozzle tip extending outwardlyfrom said nozzle, said throughbore terminating in a nozzle orifice fordischarging said liquid coating material from said nozzle orifice as abead of liquid coating material; selectively directing said pressurizedair from said air delivery passageway into a plurality of air boresformed in said nozzle to operate the liquid spray device in at least twomodes of operation; at least one pressure-assisted mode of operationwherein pressurized air flows through said air bores and is directedagainst the outer periphery of said bead of liquid coating material; anda non-pressure-assisted mode of operation wherein no air flows throughsaid air bores; in each of the at least two modes of operation,intermittently opening and closing said air delivery passageway of saidliquid spray device whereby said bead of liquid coating material beingdischarged from said nozzle orifice is applied to said substrate eitheras an atomized spray pattern, a non-atomized spray pattern or as a bead;further comprising: disposing a fluid damping system in a line betweenthe supply of pressurized air and the air delivery passageway of saidliquid spray device.